The invention relates to a power plant system for utilizing the heat energy of a geothermal reservoir in combination with processes for generating and storing additional renewable forms of energy. The power plant system can be an individual system or can be operated in a network.
A plurality of possibilities for generating electricity from renewable forms of energy are known. The following systems are built primarily as onshore devices: wind power plants, run-of-river power plants, hydroelectric power plants at reservoirs, devices for utilizing ocean and tidal currents, devices for recovery of ocean thermal energy, photovoltaic devices, power plants with mirror systems as radiation concentrators such as parabolic trough systems, solar chimney power plants, combined heat and power plants operated with bio gas or hydrogen, and geothermal heating devices. In the offshore field, increasingly wind power plants, ocean wave devices, and ocean current devices are also erected more and more.
Ocean current energy and tidal energy are fluctuating forms of energy. However, because of their periodicity, they are easily predictable and therefore planning is possible. A large consumer could be adjusted with regard to its operation. On the other hand, solar radiation, wind energy, and ocean wave energy are weather-dependent forms of energy that can be predicted only with the usual uncertainty. When directly connecting these devices to the electrical power system, it is necessary in the case of deviation of the generated electricity from the actual demand to either run up power plants using fossil fuel in order to compensate the deficiency or, when an oversupply of renewable energy is present, to throttle them.
However, these fossil fuel operated power plants are not operated at optimal working point and optimal efficiency when operated in compensating operation for fluctuating sources. In comparison to the optimal operation, for each electrical energy unit more CO2 is produced and more fuel is required. Accordingly, higher costs result in comparison to continuous operation of the power plant at the optimal working point. For this reason, the operation of power plants supplied with fossil fuels in order to enable compensation of fluctuating forms of energy, in turn, limits the environmentally friendly generation of energy of regenerative power plants.
In [1] and [2] an ocean power plant concept for use in the ocean or in a coastal area having great water depths is described that uses exclusively regenerative forms of energy. In this power plant concept at least two, if possible all, regenerative energy flows available at the site are to be used. For example, when one regenerative form of energy is not available, it can be replaced by another at the same time.
When a direct connection to an electrical power system is not possible, the energy that is generated at the site is to be consumed directly by a manufacturing process and is to be stored in this way. There are very different manufacturing processes, from foodstuff production to the production of known energy carriers to processing of metals. The product of a manufacturing process is then transported by ship, or if technically possible and economical, by pipelines to a storage facility in the vicinity of the consumer. With the aid of storage devices, an energy-consuming power system becomes independent of the arrival of regenerative energy in this way. The supply on land is always ensured because the manufactured products can be always optimally adjusted by suitable storage and distribution to the actual demand.
The power plant concept disclosed in [1] and [2] propose utilization of warm surface water that has a temperature differential relative to the cold water at approximately 800 m depth. For a sufficiently great temperature differential it is possible to take work-performing energy from a heat flow. Far away from 40 degree latitude in the northern or southern direction, the temperature differential between surface water and deep water is too minimal so that it cannot be economically used. Because of the required large ocean depth for the cold water, this concept therefore cannot be used in warm shallow seas whose depths do not reach at least 700 m to 800 m.
The described submarine reverse osmosis requires a minimum depth of approximately 300 m to 500 m in order to be able to operate especially economically in comparison to other desalination methods. When the water depth is less, additional energy must be made available for the required pressure build-up for reverse osmosis. The reverse osmosis is still an interesting method for desalinating sea water. Drinking water preparation and desalination of water are very important but also energy-intensive processes that gain in importance. The public becomes more and more aware of the worldwide depletion of drinking water.
In shallow seas such as the North Sea, the concepts disclosed in [1] and [2] can use only ocean waves, ocean and tidal currents, wind and solar radiation. The disclosed concepts of ocean thermal energy recovery by utilizing warm surface water and cold deep water is not possible in this connection.
Since however installation sites for regenerative energy devices on densely populated land decrease further, for example, by utilizing the already present suitable areas for wind energy or because of political resistance against wind power and biogas devices, it is necessary to utilize increasingly shallow sea areas near the coast.
Since the division of the shallow seas such as the North Sea by the adjoining countries into zones, oil and natural gas deposits and other possibly present natural resources can be used only by the countries to which the zone is assigned. Since approximately 1970, the shallow seas such as the North Sea are intensively industrially used and the fossil energy deposits have been exploited largely since. The accidents that occur during exploration and distribution of oil lead to significant contamination of the sea water and entire coastal areas (see current newspaper). The coastal countries that are not participating in extraction of oil and natural gas must suffer involuntarily the disadvantages and must cover the resulting high costs.
When directly connecting power plant systems that utilize different forms of renewable energy such as wind or solar radiation, there are significant deviations between generation and consumption. If it is desired to no longer use fossil fuel operated power plants for compensation, it is necessary to provide storage devices connected to the power system.
For storage purposes, concepts that produce electrolytically hydrogen with electricity derived from renewable forms of energy and convert hydrogen in fuel cells or in gas-combusting turbines to electricity are obvious and have been known for some time [3].
The disclosure in [4; see pages 26ff] describes the combination of a compressed air storage device with a natural gas turbine power plant in connection with wind energy devices. When a large supply of electricity from wind energy is present while electrical consumption through the power system is low, the oversupply is used for operating an air compressor unit. The compressed air is stored in an underground cavity such as salt cavern. The compressed air storage is thus the equivalent of a pumped storage power plant. When the demand for electricity is greater than the supply derived from wind energy, the compressed air and the natural gas are burned together in a turbine. The energy potential that is stored in the compressed air, after subtracting the conversion losses, can thus be used again and can be made available already within a short period of time. Such a device is capable of providing electrical compensation and regulating energy within less than 15 minutes. These storage contents are sufficient for a removal lasting several hours up to several days, depending on the configuration of the storage device.
In regard to this concept for air compression, there is presently no solution for use of the occurring heat loss that results during compression. In order to still enable an efficient use of compressed air, a combination with the gas turbine is therefore viewed as beneficial. The combination of the compressed air storage device with a gas-fueled combustion turbine in connection with wind energy devices enables a significant reduction of the CO2 emission. However, fossil fuel, that is available only in limited quantities and whose combustion leads to pollutants such as NOx and CO2 released into the environment, is still used.
For a very fast storage of electrical energy (within the millisecond range up to several hundred seconds), electric coil banks and capacitor banks, used for reactive power compensation, have been known for a long time. The magnetic storage device with supra-conducting coils [4, pages 162ff] is further developed in research and new high-performance capacitors [4, pages 150ff] are being tested today in connection with small applications such as flashlights with solar cells to the use in automobiles for the recovery of energy. Rechargeable electric battery systems (electrochemical secondary elements) have also been used for a long time.
Also, the known flywheel storage principle [4, pp. 178ff] in combination with an electrical machine has also been developed further. It is used in vehicles as well as in stationary energy devices. It can release within a few seconds the entire energy contents.
New concepts have been developed which use metals such as silicon [5] or aluminum [6] as universal energy carriers. For example, after the manufacture of pure silicon, the energy stored through the manufacturing process can be recovered in various processing steps with the aid of nitrogen and water via intermediates ammonia and hydrogen [5, pages 7ff].
In addition to solar chimney power plants that provide approximately constant power output, the only regenerative form of energy that is available continuously on demand in accordance with the corresponding demand of the consumer is geothermal energy.
Solar chimney power plants require extensive installation areas and will be built presumably only in desert areas far away from densely populated areas. In [12], a combination of a solar chimney power plant in connection with a solar thermal power plant is described. In this connection, the solar chimney power plant is employed for removing lost heat of the solar thermal system that can now be operated with a single water filling of the cooling system continuously. The extracted energy can be transmitted by using high-voltage power lines over land at economically acceptable conditions into the densely populated areas.
In contrast to this, the geothermal energy can be used basically at any desired location. In upper layers, up to approximately 20 m, the solar radiation has an effect on the soil temperature. In some regions of the earth, the first meters can be heated by solar radiation to temperatures of 50° C. or, conversely, can be cooled in winter to the freezing point and below. Accordingly, a temperature course results that depends only on the season. The solar thermal energy that is stored within the soil can be utilized, for example, by using horizontal geothermal collectors in connection with heat pumps for heating buildings. This energy is referred to generally as near-surface geothermal energy.
The combination of solar collectors on the roof with devices for utilizing near-surface geothermal energy by geothermal collectors or geothermal probes and heat pumps is known. It is even possible to store the heat energy collected in summer by means of solar collectors with the aid of heat exchangers within the soil at minimal depth and to use a portion of this energy again for heating starting in fall [7, pages 89ff]. Generating electricity is not provided for in these concepts.
Underground water currents, aquifers that conduct warm water or hot water, and soils that are heated by volcanic activity are used directly for heating and for producing electricity. The geological and technical principles are described in detail in [7] and [8].
Aside from the near-surface geothermal energy there is also heat energy in the deep underground. It originates according to [7, pages 9ff] from three different sources:                It is stored energy that originates from the gravitational energy that was released during formation of the earth.        It is a residue from the primary heat present before the formation of the earth.        It is generated by the decomposition of radioactive isotopes in the earth's crust. This heat is stored in the earth as a result of the minimal heat conductivity of rock.        
The heat flow that results therefrom is given at 63 mW/m2 [7, page 40]. This energy flow is referred to as deep geothermal energy. For an initial coarse estimation of the temperature increase in the deep underground of the continental crust 30° K. per km can be assumed [7, page 34]. This heat reservoir is available at any location on earth.
The method for using the heat of the deep underground will be described in the following. In the hot dry rock method (HDR) the cracks that occur naturally within the rock are used for forming a heat exchanger.
First, two boreholes at a spacing of a few hundred meters are driven in up to depths of 7000 m. Here a temperature of approximately 210° C. is present provided that no effects of volcanic activity exist that would further increase the temperature. Water is forced into one of the boreholes under high pressure. The water penetrates into the cracks that are naturally present in the rock. Because of the high pressure, the cracks are widened. The forces present within the rock can now cause a very minimal displacement of individual rock layers. When the pressure of the water is reduced again, the rocks remain in the new position and therefore provide permanent new widened cracks. The process of forced water penetration is repeated several times and in the end performed until the water will exit again from the second borehole with the aid of several pumps. Depending on the flow rate and the borehole depth the water has a temperature corresponding to that of the deep underground.
By a system of microphones, the size and the spatial expansion of the crack surfaces acting as a heat exchanger can be determined based on the breaking noises within the rock. Drilling can be performed up to a depth of 10 km. In this connection, it is even possible to guide the bore head horizontally in the deep underground after having reached a depth of several thousand meters [8, page 79]. Individual areas can be drilled in a targeted fashion in this way.
The technical knowledge for exploring the heat energy in deep underground has been developed considerably also due to the investigation and exploration of natural gas and oil deposits [7, page 208]. In Europe, an HDR system as a research facility is operated at the moment in Soultz-sous-Forets.
In the older literature [9, pages 150ff] an assessment of the usable energy potential by means of the HDR method shows that the available heat energy decreases due to the gradual cooling and as a result of the minimal supply of heat flow of the rock so that finally it can no longer be utilized economically. Reheating of a heat potential that has been exploited over several days can take up to several decades. Also, potential assessments [7, page 214] are based on a utilization duration of a geological area of 100 years. Subsequently, the exploitation of additional heat energy quantities is no longer considered economical.
Assessments of the exploitation of large heat energy quantities over a time period of a few decades show that for reheating possibly several hundred, even up to 1000 years are required. After exploitation of the heat of a geological reservoir, the device therefore would have to be dismantled and would have to be newly erected at a distance of a few kilometers. Dismantling of the device and the development of a new location incur new costs. In densely populated areas it is also possible that no new industrial facilities can be erected. As a consequence, the number of possible power plant locations decreases.
This suggests the conclusion that the utilization of geothermal energy according to the HDR method for the generally assumed heat flow of approximately 63 mW/m2 can be realized according to the present state of the art [7, page 212] in an environmentally friendly way but, when considering human time frames, this represents a single exploitation of an energy potential. The sustainability that is required today in power industry therefore makes such an energy exploitation questionable because future generations cannot utilize these locations for a long period of time. Even sites for power plants however are present only to a limited extent.